The present invention relates to a reflow soldering method and a reflow soldering furnace for heating a printed board on which solder paste is printed and electronic parts, such as surface-mounted devices (hereinafter referred to as "SMDs"), mounted on the printed board and soldering the electronic parts to the board.
Reflow soldering is known as a mounting technique for electrically connecting and mechanically fixing electronic parts, such as SMDs, to a printed board. In a reflow soldering process, various SMDs are mounted on a printed board, on which solder paste is printed in advance, in a manner such that their leads are in alignment with pads of a thin film circuit on the printed board. Thereafter, the printed board is introduced into a reflow soldering furnace (hereinafter sometimes referred to as "reflow furnace") and heated, whereupon the solder paste is melted so that the SMDs are soldered to the printed board.
The reflow soldering furnace for carrying out this reflow soldering process comprises a furnace body that is provided with a conveyor for conveying the printed board. In the reflow soldering furnace body, preheating zones and a main heating zone (or reflow zone), which are defined by furnace walls, arranged in the conveying direction of the conveyor. The printed board and the SMDs thereon, as to-be-heated objects, are heated by means of heating means that are provided in the zones, individually. The heating means may be conventional heating devices, such as a hot-gas applier for blowing a hot gas against each to-be-heated object and a radiant-heat applier using a far infrared heater and the like.
In the preheating zones of the reflow soldering furnace, each to-be-heated object is heated to a temperature of 120 to 170.degree. C. to ease thermal shocks on the SMDs. In the main heating zone that follows the preheating zones, the to-be-heated object is heated to a temperature of 210 to 230.degree. C., which is higher than the melting point (180.degree. C.) of solder by 30 to 50.degree. C., whereby the solder is melted. The to-be-heated object delivered from the main heating zone is subjected to natural or forced cooling so that the solder solidifies, whereupon the reflow soldering is completed.
With the advance of diversification of electronic parts such as SMDs, there is an increasing demand for printed boards that are mounted with a large number of electronic parts of various types each. Accordingly, a large number of electronic parts with different sizes (or different heat capacities) are expected to be reflow-soldered to each printed board efficiently and securely. On the other hand, there are printed boards of various sizes. In some cases, electronic parts may be mounted on large-sized printed boards with large heat capacities. In consideration of these circumstances, electronic parts are expected to be reflow-soldered to various printed boards with high efficiency and reliability.
In the conventional reflow soldering process, the entire to-be-heated object is heated in the furnace in which the temperature is raised to a level higher than the melting point of solder by means of a hot gas or a combination of a hot gas and an infrared heater. If the heater output is not high enough for large-sized electronic parts with large heat capacities, however, the temperatures of the parts and their surroundings cannot be raised with ease. In some cases, therefore, joints (solder joints) between the printed board and leads of the electronic parts may not be able to be heated to a predetermined temperature, resulting in defective soldering.
The aforementioned underheating can be compensated with an increase of the hot gas temperature or the heater output. If this is done, however, those portions of the printed board which carry no electronic parts thereon or small-sized electronic parts with small heat capacities will overheat. In such a case, the thin film circuit on the printed board may be disconnected or cracked, and the small-sized parts may possibly be damaged or lowered in properties.